The clinical course of cystic fibrosis (CF) varies considerably among patients carrying the same CF-causing gene mutation. Additional genetic modifiers may contribute to this variability. As airway inflammation is a key component of CF pathophysiology, we investigated whether major cytokine variants represent such modifiers in young CF patients. We tested 13 polymorphisms in 8 genes that play a key role in the inflammatory response: tumor necrosis factor, lymphotoxin alpha, interleukin (IL) 1B, IL1 receptor antagonist, IL6, IL8, IL10 and transforming growth factor beta 1 (TGFB1), for an association with lung disease progression and nutritional status in 329 CF patients. Variants in the TGFB1 gene at position +869T/C demonstrated a significant association with lung function decline. A less pronounced rate of decline in forced expiratory volume in 1 sec (FEV1) and forced vital capacity (FVC) were observed in patients heterozygous for TGFB1 +869 (+869CT), when compared to patients carrying either TGFB1 +869TT or +869CC genotypes. These findings support the concept that TGFB1 gene variants appear to be important genetic modifiers of lung disease progression in CF.